I am running an Exchange 5.5 server behind an ISA 2000, and using the SMTP service on the ISA server to relay the mail to/from the internet to the Exchange server. My ISA server and Exchange server do NOT have MX records, as I bring all mail in via an external filtering service. I have incoming SMTP restricted to the IP range of the extermal mail filtering service (Mailwise) as well as a small range of addresses for my domain to allow some internet-facing services to send email notifications with their SMTP services.

A portscan of the ISA server from anything except the two ranges I have described will show no SMTP service at all on the ISA server's external address. (GRC.COM's ShieldsUP shows port 25 as "Stealth")

My problem is that some servers are rejecting outgoing mail deliveries from the SMTP service on the ISA server with the error message shown in the subject of this post (Helo command rejected: need fully-qualified hostname), apparently because they cannot see anything at the address sending, or because the SMTP server is showing a name of DP2KISA.DINGLEY.COM where the mail is originating from DP2KMAIL.DINGLEY.COM (the ISA server's FQDN rather than the originating exchange server's FQDN)

First - have I identified the problem correctly as to the issues, and second, how do I get the SMTP service to show a FQDN to the receiving server. Is it as simple as making an entry in my DNS (we host our own primary DNS) for an MX record for the ISA server, or is there more necessary to to get the server to show the FQDN when doing the Helo command?

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Neither of these issues turns out to be the problem, the SMTP service itself defaults to servername only (no domain suffix) so you have to enter the FQDN in the SMTP service manually from the IIS control app.